something I came up with a few days ago
I've done this before, but I figured it might be cool to see what others come up with as well
the basic idea behind this is to practice anatomy or perhaps learn a few new things, when i did it I learned a lot about the purposes of the different pits,dips and loops of a skull, and of the different placement of the muscles

anyhow draw you characters skull in the first panel, then copy it, and paste it in the second at a low opacity, then draw your chrs head over it.
here is an example
http://www.furaffinity.net/view/4067884/

for those of you with real animal characters, this is a good way to get more familiar with their anatomy, or to try and figure out how to blend the human aspects of the skull inot it better
if you have a fictional chr, then now is a great time to do some serious boning up on any creature with a similar structure

If you guys do try this please comment with a link here, I'd love to see how it goes for you